Method of making electroacoustic transducers



F. MASSA April 14, 1964 METHOD OF MAKING ELECTROACOUSTIC TRANSDUCERS Original Filed Sept. 17, 1957 Jal April 14, 1964 F. MASSA METHOD OF MAKING ELECTROACOUSTIC TRANSDUCERS Original Filed Sept. 17, 1957 3 sheets-sheet 2 Wvg/vrom 777m- April 14, 1964 F. MAssA 3,128,532

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ATTORNEY United States Patent Oiiice 3,128,532 Patented Apr. 14, 1964 3,128,532 METHD F MAKING ELECTROACUUS'IIC TRANSDUCERS Frank Massa, Cohasset, Mass., assignor to Massa Division of Cohn Electronics, Inc.

Original application Sept. 17, 1957, Ser. No. 684,551, now Patent No. 2,967,957, dated Jan. 1t), 1961. Divided and this application Dec. 9, 1960, Ser. No. 75,028

10 Claims. (Cl. 2.9-25.35)

The invention is concerned With improvements in the method of constructing a low-cost transducer adapte-d for operating efficiently at a predetermined frequency reglon. Although the invention is not limited to the high-frequency region, it is of particular economic value when used for constructing transducers operating at the higher audible frequencies or in the ultrasonic frequency region since such transducers replace more costly conventional systems for operating in the frequency region above approximately 10,000 cycles per second. IIll-lis application is a division of my co-pending application, Serial No. 684,551, led September 17, 1957, now Patent No. 2,967,- 957, granted January 10i, 1961.

An object of the invention is to provide a method of constructing an electroacoustic transducer `having improved eciency in a desired `frequency band.

Another object of the invention is to provide ian improved method for constructing a transducer having minimum variation in resonant frequency with change in temperature.

A still further object of the invention is to provide a low-cost method of fabricating an electroacoustic transducer.

Other objects and advantages of the invention will become evident by reading the specifications which follow. The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, as Well as advantages thereof, will best be understood from the following description of several embodiments thereof when read in connection with the accompanying drawings, in which:

FIG. 1 is a perspective drawing of a transducer incorporating one form of the invention utilizing a hermetic seal type of construction.

FIG. 2 is a section taken through the line 22 in FIG. 1 and shows the internal construction of one form of the hermetic seal type of transducer.

FIG. 3 is an exploded isomeric drawing of a transducer incorporating another embodiment of the invention.

FIG. 4 is a sectional view of the transducer as shown in FIG. 3 wherein the elements in FIG. 3 have been assembled.

FIG. 5 is a plan view of a piezoelectric crystal plate element such as X-cut quartz, L-cut ammonium dihydrogen phosphate, Y-cut lithium sulphate, polarized barium titanate, or any other plate in which the application of a voltage across the thickness of the plate will cause a change in the dimensions of the plate.

FIG. 6 is a cross-sectional view of the plate element taken along line 64-6 in FIGURE 5, and illustrates the position of the electrode surfaces.

FIG. 7 is a plan view of a thin plate element of a transducer material which effectively behaves as a piezoelectric substance but is a material whose transducer properties lare produced by subjecting the material to a D.C. polarizing voltage, such as barium titanate, lead metaniobate, or any other similar transducer substance.

FIG. 8 is a cross-sectional view :of the element taken along line 8 8 in FIG. 7, and shows the relative positions of the electrodesurfaces with respect to both faces of the piezoelectric plate.

FIG. 9` shows a plan view of the opposite face of the transducer plate element as was shown in FIGURE 7 and indicates two separated electrode portions of the transducer.

FIG. 10 shows a schematic diagram for the application of polarizing voltages to the electrode surfaces of the plate element shown in FIGURES 7, 8 and 9.

FIG. l11 shows a plan view of one face of a transducer plate element indicating another type of preferred arrangement of separated electrode areas on one face of the material. The transducer material in FIG. 11 is of a polarizable type similar to the plate element indicated in FIGURE 7.

FIG. 12. is a cross-sectional view of the plate element taken along the line 1`2-12 in FIG. l1.

FIG. 13 is a plan view of the opposite face to that shown in FIG. l1 and indicates an arrangement of separated electrode surfaces for achieving increased voltage sensitivity of the transducer.

FIG. 14 shows a schematic diagram for the application of polarizing voltages to the electrode surfaces of the element shown in FIGS. 11, 12 and 13.

FIG. 15 shows `a schematic assembly 0f an elect-r0- acoustic transducer element -such as the element indicated in `FIGURE 5 aiiixed to an inert plate.

FIG. 16 schematically illustrates the bending that takes place in the assembly of FIGURE 15 Iwhen a potential in a given direction is applied to the electrode surfaces of the transducer element.

FIG. 17 schematically illustrates the reverse bending that takes place in the assembly of FIGURE 15 when the applied voltage is reversed in phase.

FIG. 18 is a graph illustrating the variation of resonant frequency with temperature of the transducer assembly generally illustrated in FIGURE 15 and shows the improvement in temperature stability which results when the inert plate of the assembly is made from a preferred type of material.

FIG. 19 shows a schematic `diagram for connecting the transducer of FIGURE 1 or FIGURE 3 to an electrical circuit to produce an increase in sensitivity and band width for the transducer characteristic.

FIG. 20 is a graph showing the relative change in response characteristic of the transducer when connected as illustrated in FIGURE 19.

FIG. 21 shows a schematic diagram of an alternate method for connecting the transducer to an electrical circuit.

FIG. 2.2 is a graph illustrating the improvement in the response characteristic .that results from the connection shown in FIGURE 2.1.

FIG. Z3 is a schematic diagram illustrating another type of electrical circuit connection which may be used with the transducer for increasing its sensitivity when operating as `a microphone.

FIG. 24 is a graph illustrating the increased sensi tivity resulting from the electrical connection illustrated in FIGURE 23.

Referring more particularly to the figures in which the same reference character will be used to illustrate the same part when it appears in different figures. The reference character 1 is the radiating surface of a transducer assembly indica-ted lgenerally by numeral T. The radiating surface y1. is shown in the cross-section of FIGURE 2. Bonded intimately to one face of the plate Y1 by means of a thixotropic adhesive or cement 2 is a piezoelectric plate element 3. lslthough the plate element 3 may be of any one of the several types of plates that will be described in connection `with :FIGURES 5, 7 and 1l, the

. structure of the element 3 described in FIGURE 7 is being illustrated. It shall be noted that in (FIGURE l, the diameter of the plate or surface ll is greater than the diameter of the element 3. The electrode surfaces 31 and 32, respectively, are electrically connected by means of the flexible electrical leads 7 and t; to the two electrical conductors 9 and 10 of the cable 1l as shown. IFor illustrative purposes, a two-conductor shielded cable 1l and the shield l2 is electrically connected to rthe housing 13 as shown. This particular cable choice and electrical connection leaves both electrical terminals of the transducer assembly T free from ground and provides a totally enclosing ground shield which is desirable under certain -applications ywhere the transducer assembly T is used as a receiver of low intensity acoustic signals; and furthermore, the transducer T is used in the vicinity of electrical equipment in which interfering electrical fields may be present.

In the construction of the transducer T the plate` element 1 is rmly attached to the housing 13 by means of adhesive or an organic or inorganic cement f4. This design permits the transducer assembly T yto be completely waterproo-fed and the transducer element 3 becomes totally enclosed Within a waterproof housing structure 13 comprising the vibratory surface plate l in combination with the housing 13. The cable 11 may be made watertight at the entrance to the housing yi3 lby any conventional means such as by applying a waterproof sealing compound 15 at the joint between the cable periphery and the opening through the housing structure 13.

In the operation of the transducer assembly T shown in FIGURES l and 2 an alternating voltage applied to the cable terminals will cause changes in `dimension in the transducer plate element 3, lwhich for the purposes of illustration, is assumed to be polar-ized barium titanate. The Variations in dimension of the transducer plate element 3, will cause bending of the radiating surface ll as will be more Ifully discussed in connection fwith the description of FIGURES l5, 16 and 17. When the resonant frequency of the applied electrical signal coincides with the natural period of yvibration of the composite sandwich plate element assembly 1 and 3, the sensitivity of the transducer will be a maximum.

If an alternating sound pressure strikes a diaphragm type radiating surface -ll of the transducer assembly T, the oscill-ation of the diaphragm surface 1 will cause corresponding stresses in the transducer plate element 3 which, in turn, will generate corresponding alternating voltages across the electrodes 31 and 32 which, in turn, will appear at the terminals 7 and 8 of the cable 11. When the frequency of the alternating sound pressure corresponds with the natural resonant frequency of the cornposite sandwich plate assembly l and 3` the output volta-ge from the transducer assembly T will be a maximum.

`In FIGURES 3 and 4 the housing structure In is entirely separate from the transducer element 3. The transducer assembly T comprises an inert plate 3@ which is bonded by means of the cement 2 to the transducer element 3 which, for purposes of illustration, is assumed to be polarized barium titanate. The electrode surfaces 37 and 34 are connected by means of flexible leads 7 and 8 to the center terminal 1S 4and the shield l9, respectively, of the cable 25. In this illustration, a single conductor shielded cable `25 is shown .in which the shield 19 serves as one terminal and the center conductor as the other. The cable 2S passes through an opening 26 through a metallic end plate Ztl and the shield t9 is preferably secured to the walls of the open-ing 26 by means of solderv ing to establish electrical connection between the shield 19 and the end plate Ztl. -A metallic screen 21 is placed inside the housing structure te and covers the opening Q2 which is placed through the housing structure lr6 to permit the passage of sound Waves. The transducer assembly platte of the transducer plate element 3 and inert plate 30, such as aluminum, nickel, copper and the related alloys of these metallic elements, will be set into free d vibration when alternating voltages are supplied through the cable Z5 to the electrodes 31 and 32.

When the frequency of the applied alternating voltages correspond with the natural frequency of vibration of the transducer plate assembly, the sensitivity will be a maximum. For the natural mode of Ivibration of a free plate, it is well known that the center portion moves out of phase with the peripheral portion of the plate surface. If the entire face of such a vibrating free plate were permitted to radiate sound into the medium, there would be a reduction in sensitivity of the structure due to the cancelling effect of the out-of-phase vibrations between the center portion of the transducer plate assembly 3 and 3th and the outer surface of the assembly 3 and 30. In order to prevent this phase interference, a resilient washerlike member ll7, such as cork, has been provided, which offers negligible impedance to the free `vibration of the transducer plate assembly, but effectively shields the outer surface larea of the plate 3@ so that only the center portion of the assembly 3 and 30l is exposed to the at- -mosphere through the opening 22. The natural frequency of vibration of the transducer plate assembly 3 and 36 is a function of the thickness and diameter of the assembly and also a function of the thickness of the cement joint 2. The resonant frequency of the plate assembly is inversely proportional to the `square of the diameter of the plate assembly and directly proportional to the thickness of the plate.

In many applications of a transducer, it is desired that the maximum sensitivity occur at the same frequency, and it has been found possible to reduce the cost of production and maintain close limits in frequency by resorting to the use of a small weight 23 which is bonded to the plate 3f) by means of the cement 2. The weight 23 is preferably in the form of a small disc which is selected such that its loading effect reduces the resonant frequency of the plate assembly 3 and 30 by the desired amount. Thus, the preferred method of constructing an electroacoustic transducer assembly comprises the steps of setting the dimensional tolerances on the components of the transducer element such that the resonant frequency variations among the assembled transducer elements are such that the resonant frequencies lie above the desired frequency of operation; Calibrating the transducer structure to determine its resonant frequency and then adding a selected concentrated mass to the vibrating surface of the transducer element of such magnitude that the resonant frequency of the transducer element is reduced to the required operating value. Essentially, a bilaminar assembly is formed which includes a transducer element and an inert elastic plate, the assembly being of such dimensional size that the resonant frequency of the Vibrating portion of the assembly lies above the desired frequency of operation. Then a selected concentrated mass is axed t0 the vibrating surface of the assembly to reduce the resonant frequency of the assembly to the required operating value.

An alternative method of constructing an electroacoustic transducer containing a resonant transducer element that is required to operate at a predetermined frequency is to form the components of the transducer element of such dimensional tolerances that the resonant frequency variations among the assembled transducer elements lie below the desired frequency of operation. 'Ille transducer elements may then be calibrated to determine the resonant frequency thereof. By removing sufcient material from the periphery of the transducer elements so as to reduce the transverse dimension thereof, the resonant frequency may be raised to the desired value. Basically, the method comprises the steps of bonding a transducer element to an inert plate to `form an assembly of such dimensional tolerances that the frequency of resonance lies below a desired predetermined frequency of operation and removing sufficient material from the periphery of the assembly to raise the resonant frequency to the desired value. If a cylindrical assembly were formed, the diameter would be reduced to raise the -resonant frequency of the assembly to the desired frequency.

It will be understood that the resonant frequency of a transducer having a composite vibratile element whose resonant frequency is higher than the desired frequency of operation of the transducer may be controlled by removing material from the surface of said composite vibratile element such that the thickness dimension is reduced and the resonant frequency, determined as a function of motional impedance of the composite vibratile element, is lowered to the desired operating value.

Another method of constructing an electroacoustic transducer containing a resonant transducer element that is required to operate at a predetermined frequency comprises the steps of: setting the dimensional tolerances on the components of the transducer element such that resonant frequency variations among the assembled transducer elements are such that the resonant frequencies lie below the desired frequency of operation; Calibrating the transducer structure to determine its resonant frequency; and then removing material from t-he periphery o-f the element sufficient to `raise the resonant frequency to the desired value.

The preferred method of construction of the transducer of FIGURE 4 to result in a low-cost structure is to make a subassembly of the transducer plate structure 3 and 30 with the cable and lid structure 25 and 20, respectively, providing a resilient ring 24 of a material such as foam rubber between the end plate 20 and the element 3 as shown in FIGURE 4. This sub-assembly is then dropped into the opening in housing 16 and the outer edge 27 is crimped to completely finish the transducer assembly T.

The completed transducer will operate as a sound transmitter or a microphone and the optimum efficiency will occur at the natural resonance of the transducer element plate assembly 3 and 30 which is determined by the dimensions of the element 3 and plate 30, as well as by the magnitude of the weight member 23.

FIGURES and 6 illustrate a plan and side View of a piezoelectric crystal plate showing electrode surfaces 28 and 29, such as slated or evaporated silver, gold, copper and the like, which are in the form of continuous discs substantially covering practically the entire plane faces of the crystal element 3. The piezoelectric element 3 may be any one of the well known cuts of piezoelectric crystals in which an applied voltage across the electrode surfaces 2S and 29 will cause a dimensional change in the dimensions of the element 3 such as, for example, L-out ammonium dihydrogen phosphate, Y-cut lithium sulphate, or polarized barium titanate. If a piezoelectric element 3 is intimately bonded to an inert plate 30 of semielastic material as illustrated in FIGURE 15, and if lalternating voltages are applied to the electrode surfaces 2S and 29, the composite structure of FIGURE l5 will bend alternately as illustrated in FIGURES 16 and 17 if the polarity of the applied voltages is reversed between electrode surfaces 28 and 29. The 'basic reason why the composite structure bends is due to the fact that the applied voltage lacross the piezoelectric plate causes an alternate contraction and expansion of the element diameter. These changes in dimensions of the element 3 would cause a buckling of the assembly illustrated in FIGURE which effectively causes an oscillation of the assembly along the element 3 at right angles to the plane of the composite plate stnucture.

If a piezoelectric material is chosen whose activity results after D.C. polarization of the substance such as the class of substances which include barium titanate and lead metaniobate, it is possible to increase the voltage sensitivity of the transducer structure when used as a microphone by applying electrode surfaces in disconnected areas over ,the surface of the element 3 and by polarizing the material in a prescribed manner with reference to the disconnected electrodes. FIGURES 7, 8 and 9 illustrate one method that has been found advantageous for effecting an increase in the voltage sensitivity of a transducer assembly T when used as a microphone. A barium titanate element 3 is covered with an electrode surface 2 on one of its faces as shown in FIGURE 7. On the opposite face of the plate two symmetrical areas of electrodes 31 and 32 are applied, as shown in FIGURE 9, leaving a margin 33 along one diameter of the element 3 as shown. If this element 3 is polarized by applying a D.C. potential of magnitude of about 2V, where V is an arbitrary magnitude of voltage, between electrodes 31 and 32 as illustrated in the circuit diagram of FIGURE 10, and if the center tap voltage -l-V is applied to the electrode 28, then each half of the crystal element 3 will be polarized in opposite polarity as illustrated schematically by the plus and minus markings in FIGURE 8. With this type of polarization, the voltage generated across the electrodes 31 and 32, when the element 3 is assembled into a composite assembly such as illustrated in FIGURE 15, will be twice the value that would be realized for the configuration of the elements indicated in FIGURES 5 and 6.

In FIGURES 11, 12 and 13, another embodiment is shown illustrating a multiple of subdivision of electrode surfaces in which four quadrant sections are effectively separately polarized as illustrated in the schematic wiring diagram of FIGURE 14. On one face of the element 3 are two separated electrodes 31 and 32 as shown in FIG- URE 11. This configuration is equivalent to the configuration illustrated in FIGURE 9. The opposite face of element 3 contains four symmetrical electrode areas 34, 35, 36 and 37. A pair of electrodes 36 and 35 are electrically connected by the conducting strip 38 as shown. FIGURE 14 schematically shows the application of the D.-C. polarizing voltage across the electrode surfaces 34, 35, 36 and 37. If the electroded surface illustrated in FIGURES 11 and 13 is substituted for the electroded surface shown in FIGURES 7 and 9 and the same composite plate assembly is made as illustrated in FIGURE 15, then a higher voltage will be generated as the composite transducer assembly is bent because of the fact that four incremental voltages will be effectively in series for the electrode connection o-f FIGURES 11 and 13 as compared to the two incremental voltages which will appear in series for the arrangement of FIGURES 7 and 9.

In the conventional method of electrodes in which a single potential exists between the entire surface of the element 3 such as illustrated in FIGURES 5 and 6, only one increment of voltage will appear across the electrodes during the bending of the composite assembly. Therefore, it can be seen that in the case of a polarizable material, such as barium titanate, it is possible to subdivide the electrodes and to effectively break up the transducer plate into multiple pairs of elements and to polarize these pairs of elements in such fashion that the alternating voltages generated in each of the separate areas of the element 3 will be effectively connected in series to result in increased output voltage when the structure is employed as a microphone.

In making the transducer plate assembly 3 and 30, plate 30 may be any inert elastic material and it may be either metallic or non-metallic. For transducer assemblies that are to operate in the frequency region above 10,000 cycles per second, it has been found that the use of an aluminum plate as the inert member of the sandwich assembly gave satisfactory results. It has been also found that if the thickness of the aluminum plate 30 is made equal to or greater than the thickness of the barium titanate element 3, the entire thickness of the barium titanate will be in tension when it is on the convex side of the bent assembly, as illustrated in FIGURE 16, and in compression when it appears on the concave side as illustrated in FIG- URE 17. If the thickness of the aluminum plate 30 is less than the thickness of the barium titanate element 3, a portion of the barium titanate element 3 nearest the center of the composite assembly will be of opposite stress to the outer portion of the element when the assembly is deformed and under this condition the voltage generated in that inner portion of the plate would be of opposite phase to the voltage generated in the outer portion of the barium titanate element 3, thus causing a reduction in sensitivity.

By using an aluminum plate in combination with a barium titanate element 3, it has been found possible to minimize the Variation of resonant frequency of the assembly with temperature. Curve 39 in FIGURE 18 shows the relative change in resonant frequency with temperature of a composite assembly of a barium titanate element bonded to an unpolarized plate of barium titanate. Curve 40 shows the relatively negligible variation in resonant frequency with changes in temperature for the composite assembly of a barium titanate element bonded to an aluminum plate.

FIGURE 19 illustrates a circuit arrangement for operating the transducer of FIGURE 1 or FIGURE 3 as a loud speaker and to achieve an increase in sensitivity over a wider frequency band of operation. The structure 4l represents the transducer. An inductance 42 is connected in series with the transducer 41 and the magnitude of the inductance is selected so that its reaetance is equal to the reactance of the transducer 41 at its resonant frequency of operation. The tuning effect of the series inductance will change the response curve 43 of the individual transducer to the response curve 44 as shown in FIGURE 20.

FIGURE 21 shows an alternate tuning arrangement in which the inductance 42 is connected in parallel with the transducer 41. If the reactance of the inductance 42 is made equal to the reactance of the transducer 41 at the resonant frequency of the transducer, response curve 45 results from the tuned circuit as compared to the original response curve 43 for the transducer alone. The circuit connection of FIGURE 21 results in a higher electrical impedance at resonance than the connection of FIGURE 19. Therefore, the Voltage output from the transducer 41 when used as a microphone with the circuit connection indicated in FIGURE 21 will be increased as illustrated by curve 45 in FIGURE 22.

In FIGURE 23 is shown another circuit arrangement in which condenser 46 is placed in series with the transducer 41 and an inductance 47 shunts the combined series elements as illustrated. This circuit connection has the effect of stepping up the impedance higher than results from the simple shunt circuit shown in FIGURE 21, and as a result, the output of the transducer 41 when used as a microphone may be increased appreciably as illustrated by curve 48 in FIGURE 24 as compared to the sensitivity curve 43 of the transducer operating by itself.

It shall be noted tha-t the transducer herein described has numerous applications as a microphone, receiver or transmitting structure. In particular, the transducer may be an element used in a remote sensing device, such as :a television frequency selector. The transducer is initially activated by a continuous signal or pulse generated by striking a resonant reed or tuning fork. The remote signal is then converted into an electrical signal by the transducer which initiates a stepping relay or solenoid which operartaes a frequency band selector switch.

Another application of the transducer-receiver is to incorporate the device with a multiple geared stepping relay having variable contact elements in or-der to develop a sequential program system. This system in turn would establish a combination sequence which has been adapted for remote door opening `devices and the like. Another modification is to use two or more transducers having different frequency bands, thereby materially increasing the number of combinations.

r[he ultrasonic frequency range is generally considered in the frequency region in excess of 10,000 cycles per second. The upper frequency range may exceed a quarter of a million cycles per second although one hundred thousand cycles per second alfords a practical range. In the instant invention, -I have found a preferred range of twenty to twenty-seven thousand cycles per second; although the range may be extended or lowered depending on the particular application.

My invention has been described in connection with several embodiments which have been chosen to illustrate the basic ideas; however, it will be obvious to those skilled in the art lthat numerous deviations will be possible from the specific details shown, and therefore, the invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

What is claimed is:

1. The method of constructing an electroacoustic transducer `operable at a predetermined frequency which includes the steps of forming a bilaminar assembly including a transducer element and an inert elastic plate of such dimensional size that the resonant frequency of the vibrating portion of the assembly lies above the desired frequency of operation; and affixing a selected concentrated -mass to the vibrating surface of the assembly of such magnitude that the resonant frequency of the assembly is reduced to the required operating value.

2. The method of controlling Ithe resonant frequency of an electroacoustic transducer comprising the steps of bonding a first plate to a vibratile element to form an assembly whose resonant frequency is actually higher than the desired frequency of operation of the transducer, and `attaching a concentrated mass of predetermined magnitude to the assembly to reduce the actual resonant frequency of the assembly to the desired operating value.

3. The method of constructing an electroacoustic transducer comprising the following steps: bonding a transducer element to an inert plate to form an assembly of such dimensional tolerances that the frequency of resonance lies below the desired predetermined frequency of operation; and removing suticient material from the periphery of the assembly to raise the resonant frequency to the desired value.

4. The method of constructing an electroacoustic transducer that is required to operate at a predetermined frequency comprising the yfollowing steps: bonding a rst plate to a second plate having the property of being made piezoelectric by the application of a direct current polarizing potential to form a composite disc having a resonant frequency which lies above the desired frequency of operation; and removing material from the surface of said disc sufficient to reduce the resonant frequency to the required operating value.

5. The method -of controlling the resonant frequency of an electroacoustic transducer element which includes a composite vibratile element whose resonant frequency is higher than the desired frequency of operation of the trans-ducer comprising the step of removing material from the surface of said composite vibratile element such that the thickness dimension is reduced and the resonant frequency determined as a function of motional impedance of the composite vibratile element is lowered to the desired operating value.

6. The method of controlling the resonant frequency of an electroacoustic transducer comprising the steps of bonding a first plate to a second vibratile plate to form a composite assembly whose resonant frequency is lower than the desired frequency of operation of the transducer, and removing an amount of material from the periphery of the composite assembly as determined from the measurement of the motional impedance of Ithe composite assembly sufficient to raise the resonant frequency to the required operating value.

7. The method of constructing an electroacoustic transducer assembly operable at a predetermined frequency in the frequency region above 10,000 cycles per second comprising the steps of bonding an aluminum plate to a transducer element comprising barium titanate to form a unitary disc of such dimensional size that the resonant frequency of the vibrating portion of the unitary disc lies above the predetermined frequency of operation, and attaching a compensating `disc of predetermined mass to the central area of said vibrating portion of sai-d unitary disc to reduce the resonant frequency of the unitary disc to said predetermined frequency.

8. The method of constructing an electroacoustic transducer having a bending resonance mode compris-ing the steps of bonding an inert plate to a transducer element to form a bilaminar disc of such dimensional size that the natural frequency of the vibrating portion of the bilanlinar disc lies above the desired frequency of operation, and axing a concentrated mass of predetermined magnitude to Ithe center of the vibrating portion of the bilaminar disc to reduce the resonant frequency thereof to a required operating 'value.

9. The method of constructing an electroacoustic transducer comprising the steps o-f bonding an tiner't material to a piezoelectric element to form a composite transducer member having a resonant frequency which is greater than the desired frequency of operation, and removing suicient material from the surface of said inert material to reduce .the resonant frequency of the composite transducer member to the require-d operating frequency.

10. The method of constructing an electroacoustic transducer comprising the steps of bonding a barium titanate element to an aluminum plate Which lis at least as thick as the barium titanate element to form a unitary assembly of such dimensional size that the resonant frequency of the vibrating portion of the unitary assembly lies above a predetermined frequency of operation, and attaching a compensating `disc of predetermined loading effect to the center of the vibrating portion to reduce the resonant frequency by a predetermined amount.

References Cited in the tile of this patent UNITED STATES PATENTS 1,869,160 Marrison July 2-6, 1932 2,765,765 Bigler Oct. 9, 1956 2,870,521 Rudnick Ian. 27, 19159 

1. THE METHOD OF CONSTRUCTING AN ELECTROACOUSTIC TRANSDUCER OPERABLE AT A PREDETERMINED FREQUENCY WHICH INCLUDES THE STEPS OF FORMING A BILAMINAR ASSEMBLY INCLUDING A TRANSDUCER ELEMENT AND AN INERT ELASTIC PLATE OF SUCH DIMENSIONAL SIZE THAT THE RESONANT FREQUENCY OF THE VIBRATING PORTION OF THE ASSEMBLY LIES ABOVE THE DESIRED FREQUENCY OF OPERATION; AND AFFIXING A SELECTED CONCENTRATED MASS TO THE VIBRATING SURFACE OF THE ASSEMBLY OF SUCH MAGNITUDE THAT THE RESONANT FREQUENCY OF THE ASSEMBLY IS REDUCED TO THE REQUIRED OPERATING VALUE. 